U.S. patent application number 12/681718 was filed with the patent office on 2010-08-26 for method for producing group iii nitride crystal substrate, group iii nitride crystal substrate, and semiconductor device using group iii nitride crystal substrate.
This patent application is currently assigned to PANASONIC CORPORATION. Invention is credited to Takeshi Hatakeyama, Kouichi Hiranaka, Hisashi Minemoto, Osamu Yamada.
Application Number | 20100213576 12/681718 |
Document ID | / |
Family ID | 40549046 |
Filed Date | 2010-08-26 |
United States Patent
Application |
20100213576 |
Kind Code |
A1 |
Hiranaka; Kouichi ; et
al. |
August 26, 2010 |
METHOD FOR PRODUCING GROUP III NITRIDE CRYSTAL SUBSTRATE, GROUP III
NITRIDE CRYSTAL SUBSTRATE, AND SEMICONDUCTOR DEVICE USING GROUP III
NITRIDE CRYSTAL SUBSTRATE
Abstract
Disclosed is a method for producing a group III nitride crystal
substrate. A group III nitride crystal is formed by a growth method
using a flux. The group III nitride crystal substrate is heat
treated at a temperature equal to or higher than the lowest
temperature at which the flux contained inside the group III
nitride crystal substrate through intrusion into the crystal during
the crystal formation can be discharged to outside the group III
nitride crystal substrate, and equal to or lower than the highest
temperature at which the surface of the group III nitride crystal
substrate is not decomposed.
Inventors: |
Hiranaka; Kouichi; (Ehime,
JP) ; Minemoto; Hisashi; (Osaka, JP) ;
Hatakeyama; Takeshi; (Ehime, JP) ; Yamada; Osamu;
(Ehime, JP) |
Correspondence
Address: |
HAMRE, SCHUMANN, MUELLER & LARSON P.C.
P.O. BOX 2902
MINNEAPOLIS
MN
55402-0902
US
|
Assignee: |
PANASONIC CORPORATION
Osaka
JP
|
Family ID: |
40549046 |
Appl. No.: |
12/681718 |
Filed: |
October 8, 2008 |
PCT Filed: |
October 8, 2008 |
PCT NO: |
PCT/JP2008/002834 |
371 Date: |
April 5, 2010 |
Current U.S.
Class: |
257/615 ;
257/E21.09; 257/E21.53; 257/E29.089; 438/16; 438/478 |
Current CPC
Class: |
H01L 33/0075 20130101;
H01S 5/0206 20130101; B82Y 20/00 20130101; H01L 21/0242 20130101;
C30B 29/406 20130101; H01S 5/32341 20130101; H01L 21/0254 20130101;
C30B 29/403 20130101; H01L 21/0262 20130101; H01S 2304/04 20130101;
H01S 5/22 20130101; H01S 5/34333 20130101; C30B 9/10 20130101; H01L
21/02625 20130101; H01L 21/02458 20130101 |
Class at
Publication: |
257/615 ;
438/478; 438/16; 257/E29.089; 257/E21.09; 257/E21.53 |
International
Class: |
H01L 29/20 20060101
H01L029/20; H01L 21/20 20060101 H01L021/20; H01L 21/66 20060101
H01L021/66 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 9, 2007 |
JP |
2007-262815 |
Claims
1. A method for producing a group III nitride crystal substrate
comprising: Forming a group III nitride crystal by a growth method
using a flux; Slicing the group III nitride crystal to form a group
III nitride crystal substrate; and Heat treating the group III
nitride crystal substrate at a temperature equal to or higher than
a lowest temperature at which the flux contained inside the group
III nitride crystal substrate through intrusion into the crystal
during the crystal formation can be discharged to outside the group
III nitride crystal substrate, and equal to or lower than a highest
temperature at which a surface of the group III nitride crystal
substrate is not decomposed.
2. The method for producing a group III nitride crystal substrate
according to claim 1, further comprising removing, by cleaning the
group III nitride crystal substrate, the flux discharged by the
heat treatment to outside the group III nitride crystal substrate
and attached to the substrate.
3. The method for producing a group III nitride crystal substrate
according to claim 1, further comprising performing planarization
of planarizing the surface of the group III nitride crystal
substrate after the heat treatment.
4. The method for producing a group III nitride crystal substrate
according to claim 3, wherein the performing planarization
comprises at least one of or a combination of two or more of
polishing, dry etching and wet etching to planarize the surface of
the group III nitride crystal substrate in such a way that a
surface arithmetical mean roughness of the group III nitride
crystal substrate is 0.1 nm to 5 nm.
5. The method for producing a group III nitride crystal substrate
according to claim 1, wherein the group III nitride crystal is
sliced, in the slicing, in such a way that a thickness of the group
III nitride crystal substrate is 200 .mu.m to 800 .mu.m.
6. The method for producing a group III nitride crystal substrate
according to claim 1, wherein in the crystal formation, used is one
of a flux comprising at least one of an alkali metal and an alkali
earth metal and a flux comprising ammonia in a supercritical
state.
7. The method for producing a group III nitride crystal substrate
according to claim 6, wherein a flux comprising sodium is used.
8. The method for producing a group III nitride crystal substrate
according to claim 7, wherein in the heat treating, a heat
treatment is performed at 883.degree. C. or higher and 1200.degree.
C. or lower.
9. The method for producing a group III nitride crystal substrate
according to claim 6, wherein a flux comprising ammonia in the
supercritical state and a mineralizer is used in the crystal
formation, and the ammonia and the mineralizer contained inside the
group III nitride crystal substrate are discharged to outside the
group III nitride crystal substrate in the performing heat
treatment.
10. The method for producing a group III nitride crystal substrate
according to claim 1, wherein the group III nitride is a compound
comprising nitrogen and gallium.
11. The method for producing a group III nitride crystal substrate
according to claim 1, further comprising optically examining a
macro-defect amount after the heat treatment.
12. The method for producing a group III nitride crystal substrate
according to claim 11, wherein an area ratio between defective
portions and normal portions of the group III nitride crystal
substrate as examined from a principal surface side of the group
III nitride crystal substrate is defined as a macro-defect
amount.
13. A group III nitride crystal substrate produced by the method
for producing a group III nitride crystal substrate according to
claim 1, wherein a flux atomic concentration on the surface of the
substrate and in a vicinity of the surface of the substrate is
lower than a flux atomic concentration in the substrate portion
more inner than the surface of the substrate and the vicinity of
the surface of the substrate.
14. The group III nitride crystal substrate according to claim 13,
wherein the macro-defect amount is capable of being optically
examined.
15. The group III nitride crystal substrate according to claim 14,
wherein the macro-defect amount is an area ratio between the
defective portions and the normal portions of the group III nitride
crystal substrate as examined from the principal surface side of
the group III nitride crystal substrate.
16. The group III nitride crystal substrate according to claim 14,
wherein the macro-defect amount is 1% or less.
17. A semiconductor device produced by forming, on a group III
nitride crystal substrate produced by the method for producing a
group III nitride crystal substrate according to claim 1, a group
III nitride crystal semiconductor layer and a group III nitride
crystal semiconductor element disposed on the group III nitride
crystal semiconductor layer.
18. The semiconductor device according to claim 17, wherein the
group III nitride crystal semiconductor element is one of a laser
diode and a light-emitting diode.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
group III nitride crystal substrate by using a flux growth method,
the group III nitride crystal substrate and a semiconductor device
using the group III nitride crystal substrate.
BACKGROUND ART
[0002] Group III nitride crystal semiconductors such as a gallium
nitride (GaN) crystal semiconductor are attracting attention as the
materials for blue light or ultraviolet light-emitting
semiconductor elements. Blue light-emitting laser diodes (LDs) are
applied to high-density optical discs or high-density displays, and
blue light-emitting diodes (LEDs) are applied to displays or
illumination. Ultraviolet LDs are expected to be applied to
biotechnology and the like, and ultraviolet LEDs are expected to be
applied as ultraviolet light sources for fluorescent lamps.
Further, recently applications of the group III nitride crystal
semiconductors to high-frequency high-power devices have also been
investigated.
[0003] A gallium nitride substrate, which is one of the group III
nitride crystal substrates for use in LDs and LEDs, is usually
formed by vapor phase epitaxial growth (for example, the HVPE
method: the hydride vapor phase epitaxy method). The apparatus used
for the HVPE method has a quartz reaction tube and an electric
furnace equipped with a resistance heater for heating the quarts
reaction tube. To the quartz reaction tube, a first gas
introduction port, a second gas introduction port and an exhaust
gas port are connected. From the first gas introduction port, a
mixed gas composed of hydrogen chloride gas and hydrogen gas is
introduced. From the second gas introduction port, a mixed gas
composed of ammonia gas and hydrogen gas is introduced. In a
reaction chamber, a source port of a Ga starting material is
disposed. Hydrogen chloride is introduced from the first gas
introduction port into the source port of the Ga starting material
and gallium chloride is generated. The gallium chloride and the
ammonia introduced from the second gas introduction port are
reacted with each other, and thus a gallium nitride crystal is
grown. The gallium nitride is grown on a substrate disposed in the
quartz reaction tube and heated with the resistance heater.
[0004] As the substrate, usually a sapphire substrate is used. The
dislocation density of the crystal obtained by this method is
usually 10.sup.8 cm.sup.-2 to 10.sup.9 cm.sup.-2, and hence the
reduction of the dislocation density is an important problem (for
example, JP2000-12900A).
[0005] Alternatively, instead of vapor phase epitaxial growth,
methods for growing crystals in liquid phase have also been
investigated. However, the nitrogen equilibrium vapor pressure at
the melting point of the crystal of a group III nitride such as GaN
is 10000 atm (10000.times.1.013.times.10.sup.5 Pa) or more.
Accordingly, it has been generally accepted that, for the purpose
of growing GaN in the liquid phase, the conditions set at
1600.degree. C. and 10000 atm (a high-temperature high-pressure
growth method) are required (for example, Journal of Crystal
Growth, Vol. 178, (1997), pp. 174 to 188). In a growth method that
is performed under such high-temperature high-pressure conditions,
the space that can be pressurized is extremely narrow, and it is
difficult to form in such a narrow space a crystal having a large
area of 2 inches or more required for production of a device as a
final product. Additionally, a large high-temperature high-pressure
synthesis apparatus for producing a large area substrate
impractically leads to an increase in cost.
[0006] Recently, in a nitrogen gas atmosphere, a mixture composed
of Ga as a starting material and sodium (Na) as a flux is melted at
800.degree. C. and 50 atm (50.times.1.013.times.10.sup.5 Pa), and
single crystals having a maximum crystal size of about 1.2 mm have
been obtained by using the resulting melt, with a growth time of 96
hours (for example, JP2002-293696A). The gallium nitride crystals
formed by this "sodium flux liquid phase growth method" usually
have a dislocation density of 10.sup.5 cm.sup.-2 to 10.sup.6
cm.sup.-2. In other words, high-quality gallium nitride crystals
lower in dislocation density as compared to the gallium nitride
crystals produced by the vapor phase epitaxial growth are formed
(for example, Japanese Journal of Applied Physics, Vol. 45, (2006)
pp. L1136 to L1138).
[0007] The gallium nitride crystals formed by "the sodium flux
liquid phase growth method" frequently include metals such as Na,
K, Li and Ca that are the main components of the flux enabling a
low-temperature low-pressure synthesis. The metals need to be
removed. This is because the diffusion of these metal elements over
the epitaxial film grown on the gallium nitride crystal substrate
disturbs the control of the conduction type of the epitaxial film,
and consequently the reliability of a semiconductor device such as
a laser diode or a light-emitting diode may be degraded.
[0008] Known techniques adopt a production method in which: on the
basis of the knowledge that the flux tends to deposit in the
interface between the seed crystal substrate and the crystal
undergoing liquid phase growth in the initial stage of the liquid
phase growth or the flux tends to deposit in the vicinity of the
surface of the crystal after the completion of the growth, by
performing a step of removing the flux after the completion of the
liquid phase growth and then by performing a heat treatment at a
temperature of 300.degree. C. to 800.degree. C., the flux component
in the vicinity of the both interfaces of the gallium nitride
crystal is made to deposit on the surface of the gallium nitride
crystal to be removed (for example, JP2004-224600A).
[0009] Further, there has been known a technique to prevent the
diffusion of the impurities contained in the crystal in the liquid
phase growth method of a group III nitride crystal using a flux.
This is a technique in which the impurities on the surface of the
group III nitride crystal are heat treated to be converted into
inorganic compounds, and thus the diffusion of the impurities from
the substrate is prevented (for example, JP2006-36622A).
[0010] On the other hand, there has been proposed a method for
forming a single crystal of gallium nitride (GaN), which is a group
III nitride crystal semiconductor, by applying an ammonothermal
method (for example, Journal of Crystal Growth, Vol. 287, (2006),
pp. 376 to 380). The ammonothermal method is a method in which
gallium or gallium nitride is dissolved in ammonia, a supercritical
fluid, and single crystals of gallium nitride are deposited by
varying the temperature. The supercritical fluid means a fluid in
the state exceeding the state in which the temperature limit and
the pressure limit (critical point) at which ammonia in the gas
state and ammonia in the liquid state can coexist. Such a
supercritical fluid exhibits the properties different from the
properties of the usual gas and liquid, and acts as a flux for use
in the growth of the group III nitride crystal.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0011] The present inventors precisely measured, by the secondary
ion mass spectrometry, the thermal diffusion of the sodium as the
main component of the flux inside the gallium nitride crystal
produced by the known sodium flux liquid phase growth method, and
consequently discovered that sodium remained not only in the
vicinity of the surface of the gallium nitride crystal but also
inside the gallium nitride crystal.
[0012] As described above, in the known technique, only the flux in
the vicinity of the surface of the produced gallium nitride crystal
is removed. In other words, the known technique cannot remove the
flux remaining inside the crystal, and accordingly, when the
gallium nitride crystal is sliced and thus a gallium nitride
crystal substrate is formed, there is a possibility that sodium
remains in the vicinity of the surface of the gallium nitride
crystal substrate. When the gallium nitride crystal with sodium
remaining therein is sliced to complete the production of a gallium
nitride crystal substrate for use in formation of a semiconductor
device, and the substrate is heated to a temperature of
1050.degree. C. to 1100.degree. C. and subjected to an epitaxial
film growth by a known method to form a semiconductor element, then
the remaining sodium is thermally vaporized, and thus
disadvantageously the surface of the gallium nitride crystal
substrate and the epitaxially grown film are damaged.
[0013] In the case of the gallium nitride crystal substrate formed
by the ammonothermal method, when the substrate is similarly heated
for growing an epitaxial film, the liquid-state ammonia contained
in the macro-defects in the crystal is vaporized to blow out and
thus disadvantageously the substrate is broken. In particular, the
crystal growth temperature is usually 600.degree. C. or lower in
the ammonothermal method, but the operating temperature in the
semiconductor device production is as high as 1000 to 1200.degree.
C., leading to a large temperature difference between these two
temperatures. Accordingly, when there are even a few macro-defects
that contain ammonia as condensed therein, the crystal may be
broken at a temperature for growing a thin film to produce a
semiconductor device.
[0014] Further, when an alkali-based substance is contained as a
mineralizer in ammonia, there is a possibility that an alkali metal
or an alkali earth element is similarly deposited outside the
substrate in the production of a semiconductor device to degrade
the device performances.
[0015] An object of the present invention is to remove the flux,
exerting adverse effects at the time of the epitaxial growth, from
a group III nitride crystal substrate produced by the flux growth
method, for the purpose of solving the above-described
problems.
Means for Solving the Problems
[0016] For the purpose of achieving the above-described object, the
present invention includes: a crystal forming step of forming a
group III nitride crystal by a growth method using a flux; a
slicing step of slicing the group III nitride crystal to form a
group III nitride crystal substrate; and a heat treating step of
heat treating the group III nitride crystal substrate at a
temperature equal to or higher than the lowest temperature at which
the flux contained inside the group III nitride crystal substrate
through intrusion into the crystal in the crystal forming step can
be discharged to outside the group III nitride crystal substrate,
and equal to or lower than the highest temperature at which the
surface of the group III nitride crystal substrate is not
decomposed.
[0017] According to the present invention, preferably further
included is a removing step of removing, by cleaning the group III
nitride crystal substrate, the flux discharged by the heat treating
step to outside the group III nitride crystal substrate and
attached to the substrate.
[0018] According to the present invention, preferably further
included is a planarizing step of planarizing the surface of the
group III nitride crystal substrate after the heat treating step.
The planarizing step is a treating step including at least one of
or a combination of two or more of polishing, dry etching and wet
etching to preferably planarize the surface of the group III
nitride crystal substrate in such a way that the surface
arithmetical mean roughness of the group III nitride crystal
substrate is 0.1 nm to 5 nm.
[0019] According to the present invention, in the slicing step, the
group III nitride crystal is preferably sliced in such a way that
the thickness of the group III nitride crystal substrate is 200
.mu.m to 800 .mu.m.
[0020] According to the present invention, in the crystal forming
step, preferably used is a flux including at least one of an alkali
metal and an alkali earth metal, or a flux including ammonia in the
supercritical state.
[0021] According to the present invention, a flux including sodium
is preferably used. In this case, in the heat treating step, a heat
treatment is preferably performed at 883.degree. C. or higher and
1200.degree. C. or lower.
[0022] According to the present invention, preferably, in the
crystal forming step, a flux including ammonia in the supercritical
state and a mineralizer is used, and in the heat treating step, the
ammonia and the mineralizer contained inside the group III nitride
crystal substrate are discharged to outside the group III nitride
crystal substrate in the heat treating step.
[0023] According to the present invention, the group III nitride is
preferably a compound including nitrogen and gallium.
[0024] According to the present invention, after the heat treating
step, preferably included is an examining step of optically
examining the macro-defect amount. In this case, an area ratio
between the defective portions and the normal portions of the group
III nitride crystal substrate as examined from the principal
surface side of the group III nitride crystal substrate is
preferably defined as the macro-defect amount.
[0025] The group III nitride crystal substrate of the present
invention is a group III nitride crystal substrate produced by the
above-described production method or methods, wherein the flux
atomic concentration on the surface of the substrate and in the
vicinity of the surface of the substrate is lower than the flux
atomic concentration in the substrate portion more inner than the
surface of the substrate and the vicinity of the surface of the
substrate.
[0026] According to the group III nitride crystal substrate of the
present invention, the macro-defect amount is preferably capable of
being optically examined. In this case, the macro-defect amount is
preferably an area ratio between the defective portions and the
normal portions of the group III nitride crystal substrate as
examined from the principal surface side of the group III nitride
crystal substrate. The macro-defect amount is preferably 1% or
less.
[0027] The semiconductor device of the present invention is a
semiconductor device produced by forming, on a group III nitride
crystal substrate produced by the above-described production method
or methods, a group III nitride crystal semiconductor layer and a
group III nitride crystal semiconductor element disposed on the
group III nitride crystal semiconductor layer.
[0028] In the semiconductor device of the present invention, the
group III nitride crystal semiconductor element is preferably a
laser diode or a light-emitting diode.
[0029] According to the above means, it is possible to remove the
flux, exerting adverse effects at the time of the growth of the
epitaxial film, from the group III nitride crystal substrate
produced by the flux growth method.
ADVANTAGE OF THE INVENTION
[0030] According to the present invention, when a group III nitride
crystal substrate is produced by the flux growth method, the flux
can be effectively discharged from the substrate by vaporizing to
discharge from the substrate the flux present inside the group III
nitride crystal substrate by a high-temperature heat treatment,
after the slicing step and before the growth of the epitaxial
film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a flow chart showing a method for producing a
group III nitride crystal substrate of Embodiment 1 of the present
invention;
[0032] FIG. 2A is a sectional view illustrating the step of
producing the group III nitride crystal substrate of Embodiment 1
of the present invention;
[0033] FIG. 2B is a sectional view illustrating the step next to
FIG. 2A;
[0034] FIG. 2C is a sectional view illustrating the step next to
FIG. 2B;
[0035] FIG. 2D is a sectional view illustrating the step next to
FIG. 2C;
[0036] FIG. 2E is a sectional view illustrating the step next to
FIG. 2D;
[0037] FIG. 3 is a flow chart showing a method for producing a
group III nitride crystal substrate of Embodiment 2 of the present
invention;
[0038] FIG. 4 is a graph showing the results of the secondary ion
mass spectrometry measurement for the case of the heat treatment
temperature of 900.degree. C. in Example 1 of the present
invention;
[0039] FIG. 5 is a graph showing the results of the secondary ion
mass spectrometry measurement for the case of the heat treatment
temperature of 800.degree. C. in Comparative Example 1; and
[0040] FIG. 6 is a sectional view illustrating the structure of a
semiconductor laser diode of Example 3 of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0041] The embodiments of the present invention are described with
reference to the accompanying drawings. Here, it is to be noted
that in the drawings, the same symbols are given to the same or the
corresponding members and the like.
Embodiment 1
[0042] With reference to FIGS. 1 and 2, the method for producing a
group III nitride crystal substrate of Embodiment 1 of the present
invention is described.
[0043] FIG. 1 is a flow chart showing a method for producing the
group III nitride crystal substrate of Embodiment 1 of the present
invention, and FIGS. 2A to 2E are sectional views illustrating the
steps of producing the group III nitride crystal substrate of
Embodiment 1.
[0044] First, a group III nitride seed crystal substrate 3 is
prepared in which, as shown in FIG. 2A, on a base substrate 1
having a C+ plane as the principal plane thereof, a group III
nitride seed crystal 2 is beforehand formed as a film in a
thickness of 2 .mu.m to 20 .mu.m. As the base substrate 1, any one
of the following substrates can be used: a sapphire substrate
(crystalline Al.sub.2O.sub.3) C plane, a GaAs substrate in which
the surface is a (111) plane, a Si substrate in which the surface
is a (111) plane and a SiC substrate in which the surface is a
(0001) plane. Alternatively, as the base substrate 1, a
self-supporting group III nitride seed crystal substrate may also
be used. The group III nitride seed crystal 2 is, for example, a
GaN crystal having a thickness of 2 .mu.m to 20 .mu.m. For the
growth of the seed crystals of the group III nitrides such as GaN,
the vapor phase epitaxial growth method and the liquid phase growth
method are used. As the vapor phase epitaxial growth method, the
HVPE method (hydride vapor phase epitaxy method) and the MOCVD
method (metal organic chemical vapor deposition method) can be
used. As the liquid phase growth method, the flux growth method
using an alkali metal or the like, the ammonothermal method and the
high-temperature high-pressure growth method can be used.
Alternatively, as the seed substrate 3, a self-supporting substrate
can also be used. In this case, the base substrate 1 can be
omitted. Additionally, the plane orientation (principal surface
direction) of the seed substrate may be an orientation, other than
the (0001) plane orientation, such as the (1-100) plane
orientation, the (11-20) plane orientation or the like.
[0045] Next, the seed crystal substrate 3 is placed in a crucible
as the "seed crystal" for the liquid phase growth and a flux liquid
phase growth is performed. The growth of the group III nitride
crystal by the flux liquid phase growth method is performed by
bringing a melt that includes at least one group III element
selected from gallium, aluminum and indium and a flux into contact
with the surface of the seed crystal substrate 3 in a
nitrogen-containing gas atmosphere, under the conditions that the
temperature is set at about 700.degree. C. to 1100.degree. C. and
the pressure is set at 30 atm (30.times.1.013.times.10.sup.5 Pa) to
100 atm (100.times.1.013.times.10.sup.5 Pa). As the
nitrogen-containing gas, for example, nitrogen gas or
ammonia-containing nitrogen gas is used. As the flux, an alkali
metal, an alkali earth metal, or a mixture of both of these metals
is used. Examples of the alkali metal may include at least one
selected from sodium (Na), lithium (Li) and potassium (K), namely,
one of these alkali metals or a mixture of these alkali metals.
Preferably, sodium (Na) is used. Examples of the alkali earth metal
may include calcium (Ca), magnesium (Mg), strontium (Sr) and barium
(Ba). Preferably, calcium (Ca), strontium (Sr) and barium (Ba) are
used. The alkali earth metals may be used each alone or in
combinations of two or more thereof.
[0046] When a n-type dopant or a p-type dopant is used in a melt
including the above-described at least one group III element
selected from gallium, aluminum and indium and an alkali metal, an
n-type group III nitride crystal or a p-type group III nitride
crystal can be obtained. The dopant amount is adjusted according to
the carrier concentration, which is one of the specifications of
the semiconductor device. For example, the dopant amount of a gas
dopant is adjusted by the gas dopant flow rate. Alternatively, for
a solid dopant, an intended mass of the dopant is added to the
starting materials for the group III nitride crystal growth
(inclusive of the flux). Examples of the group III nitride starting
materials may include at least one group III element selected from
gallium, aluminum and indium and an intended flux material.
Examples of the n-type dopant include Si, O, S, Se, Te and Ge.
These may be used each alone or in combinations of two or more
thereof. Examples of the p-type dopant include Mg and C. These may
also be used each alone or in combinations of two or more
thereof.
[0047] A group III nitride crystal 4 represented, for example, by
the composition formula Al.sub.xGa.sub.yIn.sub.1-x-yN (with the
proviso that 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) is formed,
as shown in FIG. 2B, so as to enclose the seed crystal substrate 3
by using such an n-type group III nitride crystal or such a p-type
group III nitride crystal. The three-dimensional structure on the
periphery and the surface of the seed crystal substrate 3 as viewed
in FIG. 2B is determined by the growth rate difference depending on
the crystallographic orientation.
[0048] The step described above is the step shown in FIG. 1 as the
crystal growing step 11.
[0049] Next, a group III nitride crystal body 7, shown in FIG. 2B,
in which the group III nitride crystal 4 is formed is fixed to a
jig by adhesion with a wax. Then, the protruding structures 9
formed on the surface and the periphery of the group III nitride
crystal body 7 are removed by grinding such as surface grinding
and/or cylindrical grinding. Thus, a group III nitride crystal body
8 as shown in FIG. 2C is obtained (the grinding step 12 shown in
FIG. 1).
[0050] Next, by using a wafer edge grinder, the group III nitride
crystal body 8 is subjected to beveling. Then, the group III
nitride crystal 4 is sliced in such a way that the thickness of the
group III nitride crystal is preferably made to be 200 .mu.m to 800
.mu.m with reference to the underside of the base substrate 1, and
thus a group III nitride crystal substrate 5 is obtained (FIG. 2D,
the slicing step 13 in FIG. 1). For the slicing step, known cutting
devices such as a multi-wire saw, an inner circumference slicer and
an outer circumference slicer are used. When the thickness of the
group III nitride crystal substrate 5 is thinner than 200 .mu.m, no
sufficient strength is obtained. On the other hand, when this
thickness is thicker than 800 .mu.m, the number of the produced
substrates is made smaller to lead to the increase of the
production cost of the group III nitride crystal substrate 5.
[0051] Next, for the purpose of removing the asperities of the
surface of the group III nitride crystal substrate 5 obtained by
slicing, the group III nitride crystal substrate 5 is polished.
Specifically, the group III nitride crystal substrate 5 obtained by
slicing is fixed by adhesion with a wax to a dummy substrate, the
adhered group III nitride crystal substrate 5 is polished in
parallel to the surface of the dummy substrate, to a predetermined
thickness and to a predetermined surface roughness. Further, the
polished group III nitride crystal substrate 5 is mirror-polished
by varying the plate material, the load, the rotation number, the
abrasive grain size and the like, so as to have a predetermined
thickness and a predetermined surface roughness, and thus a group
III nitride crystal substrate 6 is obtained (FIG. 2E, the polishing
step 14 in FIG. 1).
[0052] The surface roughness of the group III nitride crystal
substrate 6 on which a semiconductor device such as a
light-emitting diode or a laser diode is formed is such that the
average roughness Ra (Ra is defined by the arithmetical mean
roughness Ra) is preferably 0.1 nm to 5 nm. The Ra smaller than 0.1
nm makes the polishing time correspondingly longer to degrade the
productivity. The Ra exceeding 5 nm tends to increase the
probability of the occurrence of the short circuit failure
responsible for the drop of the open end voltage which is a
property of a semiconductor device.
[0053] Next, the group III nitride crystal substrate 6 obtained by
the polishing step 14 is vertically disposed on a hanger, and is
subjected to organic cleaning for the purpose of removing the
organic matter such as the waxes used in the slicing step and the
polishing step. For the organic cleaning, generally, the organic
solvents such as solvent naphtha, acetone, methanol, ethanol and
isopropyl alcohol are used in combinations. These organic solvents
are used in the order from the lower hydrophilicity to the higher
hydrophilicity. For example, organic cleaning is performed in the
order of solvent naphtha, acetone and isopropyl alcohol. Then, for
the purpose of removing the organic components, cleaning with
sulfuric acid-hydrogen peroxide mixture is performed; and
furthermore, for the purpose of removing the oxide layer of the
substrate, a buffer hydrofluoric acid treatment is performed. Here,
the above-described step is referred to as the first cleaning step
15 as shown in FIG. 1.
[0054] Next, the group III nitride crystal substrate 6 is subjected
to a heat treatment (the heat treating step 16 in FIG. 1). The
group III nitride crystal substrate 6 is placed in a quartz tube in
a manner leaning against a quartz boat. Before the heat treating
step, for the purpose of reducing the moisture and the impurity
gases such as oxygen in the furnace, a vacuum gas purge is
performed at least three times. The vacuum gas purge as referred to
herein means an operation in which the pressure inside the furnace
is reduced to a pressure of -99 kPa or lower, and then the pressure
inside the furnace is made to be 1 atm
(1.times.1.013.times.10.sup.5 Pa) with a nitrogen-containing gas.
After the vacuum gas purge, the heat treatment is performed. The
heat treatment is performed in an atmosphere in which the
nitrogen-containing gas such as either one of nitrogen gas and
ammonia gas or a mixed gas composed of both of these gases has a
predetermined flow rate and a pressure of 1 atm. The heat treatment
temperature is set at a temperature equal to or higher than the
lowest temperature (boiling point) at which the above-described
flux is vaporized and equal to or lower than the highest
temperature at which the surface of the group III nitride crystal
substrate 6 is not decomposed.
[0055] The reasons for the fact that nitrogen is essential for the
atmospheric gas are that as the atmosphere runs short of nitrogen,
the nitrogen constituting the crystal is dissociated and hence the
crystal undergoes nitrogen deficit to degrade the crystal quality
and at the same time the group III nitride crystal is decomposed.
The predetermined nitrogen gas flow rate is preferably such that
the nitrogen gas flow speed is 0.1 m/min to 10 m/min. When the
nitrogen gas flow speed is less than 0.1 m/min, the nitrogen
dissociation occurs. On the other hand, when the nitrogen gas flow
speed exceeds 10 m/min, the decomposition of the group III nitride
crystal is promoted.
[0056] The heat treatment temperature is described in detail. The
temperature equal to or higher than the lowest temperature at which
the flux is vaporized and equal to or lower than the highest
temperature at which the surface of the group III nitride crystal
substrate 6 is not decomposed means, when potassium is mainly used
as the flux, a temperature equal to or higher than the vaporization
temperature (boiling point) of potassium, namely, 774.degree. C.
and equal to or lower than 1200.degree. C. at which the
decomposition of the surface of the group III nitride crystal
substrate 6 occurs. When sodium is used as the flux, the
temperature concerned is equal to or higher than the vaporization
temperature (boiling point) of sodium, namely, 883.degree. C. and
equal to or lower than 1200.degree. C.
[0057] The heat treatment time is varied depending on the heat
treatment temperature, the epitaxial growth temperature, the
epitaxial growth time and the like, and is for example about 1 hour
to 5 hours.
[0058] The vaporization of the flux enables the effective diffusion
of the flux present in the vicinity of the surface of the group III
nitride crystal substrate 6 so as to exert adverse effects, and
enables the flux to be deposited on the surface of the group III
nitride crystal substrate 6. In other words, the flux remaining
inside the group III nitride crystal substrate 6 can be discharged
to outside the substrate 6. The vaporization of the flux into gas
increases the diffusion coefficient by a factor of a few tens or
more as compared to the diffusion coefficient in the liquid state.
Therefore, the flux gas can effectively diffuse to the surface of
the crystal substrate 6. The flux to exert adverse effects at the
time of the epitaxial growth means the fluxes present in the
defects such as the crystal grain boundaries and the inclusions in
the vicinity of the surface of the crystal substrate 6. These
fluxes are vaporized at the time of the epitaxial growth to cause
the crystal surface roughening and the crystal exfoliation. The
fluxes other than the fluxes present in the defects hardly diffuse
even at the epitaxial growth temperature. Therefore, the sodium
flux atomic concentration actually measured in the normal portions
obtained by the heat treatment performed under the above-described
conditions can be made to be equal to or less than the limit level
of the detection level of the secondary ion mass spectrometer,
namely, 2.times.10.sup.14 atoms/cm.sup.3.
[0059] As described above, the group III nitride crystal 4 is
sliced into a wafer shape to form the group III nitride crystal
substrate 6, and then before the epitaxial growth treatment, as a
device forming step, the heat treatment is performed at a
temperature equal to or higher than the lowest temperature capable
of vaporizing the flux and lower than the highest temperature at
which the surface of the group III nitride crystal substrate 6 is
not decomposed, and thus the flux in the vicinity of the surface of
the group III nitride crystal substrate 6 can be removed by
vaporization, and hence the failures due to the presence of the
flux at the time of the epitaxial growth can be suppressed.
[0060] Finally, for the purpose of removing the by-products
produced by the heat treating step 16, the group III nitride
crystal substrate 6 is subjected to a cleaning treatment (the
second cleaning step 17 in FIG. 1). Examples of the cleaning
treatment include purified water cleaning and acid solution
cleaning. As the acid solution, a hydrofluoric acid solution, a
buffer hydrofluoric acid solution, a hydrochloric acid solution, a
sulfuric acid solution and a sulfuric acid-hydrogen peroxide
mixture can be used. For example, the group III nitride crystal
substrate 6 after the heat treatment is immersed for 10 minutes in
a 3% concentration buffer hydrofluoric acid solution diluted with
purified water (49% HF aqueous solution:40% NH.sub.4F aqueous
solution=7:1), successively subjected to running purified water
cleaning for 10 minutes, and then dried with nitrogen.
Subsequently, the group III nitride crystal substrate 6 is further
immersed for 10 minutes in a sulfuric acid-hydrogen peroxide
mixture (98% sulfuric acid:30% hydrogen peroxide solution=4:1) set
at 100.degree. C. to 120.degree. C., successively subjected to
running purified water cleaning for 10 minutes, and then dried with
nitrogen.
[0061] The second cleaning step 17 as described above enables the
production of the group III nitride crystal substrate in which the
sodium flux atomic concentration in the vicinity of the surface of
the crystal substrate as evaluated with the secondary ion mass
spectrometry is lower than the flux atomic concentration inside the
substrate and is for example 2.times.10.sup.14 atoms/cm.sup.3,
which is close to the detection limit level of the secondary ion
mass spectrometer.
Embodiment 2
[0062] Next, with reference to FIG. 3, the method for producing a
group III nitride crystal substrate of Embodiment 2 of the present
invention is described.
[0063] The method for producing a group III nitride crystal
substrate of Embodiment 2 is approximately the same as the
above-described production method of Embodiment 1 as far as the
heat treating step 16, with omission of the polishing step. The
essential difference from Embodiment 1 resides in that by adding a
step of improving the surface nature degradation caused by the heat
treatment, the production yield of the finally obtained
semiconductor device is improved.
[0064] FIG. 3 is a flow chart showing the method for producing a
group III nitride crystal substrate of Embodiment 2 of the present
invention. As described above, the crystal growing step 11 to the
heat treating step 16 in FIG. 3 are approximately the same steps as
the crystal growing step 11 to the heat treating step 16 in FIG. 1.
As described above, however, the polishing step 14 before the heat
treatment shown in FIG. 1 can be omitted. In the method for
producing a group III nitride crystal substrate of Embodiment 2 of
FIG. 3, polishing is performed in the below-described planarizing
step 18.
[0065] Present Embodiment 2 is characterized in that when the
surface nature of the substrate is degraded by the discharge of the
flux gas from the substrate in the heat treating step 16, the
planarizing step 18 is performed subsequently to the heat treating
step 16.
[0066] Specifically, when the group III nitride crystal substrate
is heat treated for discharging the flux, the nitrogen
concentration on the surface of the group III nitride crystal
substrate is concomitantly decreased to form asperities on the
surface of the crystal substrate. In present Embodiment 2, the thus
formed asperities are removed by the planarization to improve the
surface nature of the group III nitride crystal substrate.
[0067] As shown in FIG. 3, first, as in Embodiment 1, the steps to
the heat treating step 16 are performed, and subsequently the
above-described surface roughness Ra of the crystal substrate is
measured. When the surface roughness Ra exceeds, for example, 5 nm
for the surface to be rough, the planarizing step 18 is performed
after the heat treating step 16 as shown in FIG. 3. Subsequently,
the second cleaning step 17 is performed, and thus the group III
nitride crystal substrate usable for the epitaxial growth at the
time of forming an element can be obtained.
[0068] The planarizing step 18 is described in detail. First, under
the condition that the group III nitride crystal substrate is
mounted on a dummy substrate, the asperities of the surface of the
group III nitride crystal substrate are removed by performing an
operation such as a mechanical polishing of the surface of the
group III nitride crystal substrate. Then, the dummy substrate is
detached, and the wax used for the adhesion of the group III
nitride crystal substrate is removed in the same manner as in the
first cleaning step 15. Further, a polishing-modified layer is
produced by the mechanical polishing or the like, and hence the
surface of the group III nitride crystal substrate is planarized by
removing the polishing-modified layer by applying a plasma dry
etching with a chlorine-based gas.
[0069] Examples of the planarizing technique other than the
above-described mechanical polishing may include wet etching with a
buffer hydrofluoric acid solution, a hydrochloric acid solution and
a sulfuric acid-hydrogen peroxide mixture. Alternatively, the
planarization can be performed by a mechanochemical polishing that
is a combination of polishing and wet etching. The mechanochemical
polishing can be performed by using, for example, colloidal silica
as an abrasive grain, and by selecting the hydrogen ion index (pH),
the pad, the load and the rotation number as predetermined.
[0070] The targeted level of the planarization is such that the
surface roughness is made not to affect the device, and in other
words the surface roughness Ra is preferably made to be 5 nm or
less.
[0071] Then, after the polishing of the group III nitride crystal
substrate, the dummy substrate is detached from the group III
nitride crystal substrate. Subsequently, for the purpose of
removing the wax used for the adhesion of the group III nitride
crystal substrate, cleaning is performed in the same manner as in
the first cleaning step 15, and further, for the purpose of
removing the ion damages suffered at the time of the plasma dry
etching, the second cleaning step 17 is performed in the same
manner as in Embodiment 1, and thus the group III nitride crystal
substrate can be provided.
[0072] For the purpose of removing the group III nitride having a
low nitrogen concentration, produced in the heat treating step as
described above, on the surface of the group III nitride crystal
substrate, the substrate surface is planarized after the heat
treating step, and hence the surface nature of the group III
nitride crystal substrate can be ensured, in addition to the fact
that the failure due to the flux, at the time of the epitaxial
growth can be suppressed by removing through vaporization the flux
in the vicinity of the surface of the group III nitride crystal
substrate.
[0073] It is to be noted that the planarizing step 18 can be
performed, as described above, after the heat treating step 16 and
before the second cleaning step 17; however, the planarizing step
18 may also be performed at the same time as the second cleaning
step 17 or after the second cleaning step 17, as the case may
be.
Embodiment 3
[0074] In above-described Embodiments 1 and 2, the flux solution
growth method using an alkali metal as the flux is used. On the
other hand, the heat treating step characterizing the present
invention can also be applied to a method for producing a group III
nitride crystal substrate using the ammonothermal method, as
Embodiment 3.
[0075] In the ammonothermal method, the gallium nitride crystal is
precipitated in ammonia being a supercritical fluid, and hence
liquid ammonia is confined inside the produced group III nitride
crystal substrate as the case may be. By performing a heat treating
step, the liquid ammonia can be vaporized to be discharged to
outside the crystal.
[0076] The heat treatment temperature required for this purpose is
about 1000.degree. C. to 1200.degree. C. Additionally, it is
necessary to set the heat treatment temperature at a temperature
equal to or lower than 1200.degree. C. that is the limit at or
below which no decomposition occurs on the surface of the group III
nitride crystal substrate. It is to be noted that the boiling point
of ammonia under atmospheric pressure is -33.degree. C., but
heating to about 1000.degree. C. or higher is required, as
described above, for the purpose of discharging ammonia as in the
gasified state to outside the crystal substrate, on the basis of
the present invention.
[0077] In the ammonothermal method, a mineralizer is used as the
case may be, for the purpose of increasing the dissolved amount of
gallium or gallium nitride in ammonia being a supercritical fluid.
In this case, there is a possibility that not only ammonia but also
the mineralizer is confined in the obtained crystal substrate. When
the heat treatment is performed on the basis of the present
invention, the mineralizer can also be discharged together with
ammonia to outside the crystal substrate, and hence the failure at
the time of epitaxial growth, due to the remaining mineralizer can
be suppressed.
[0078] Next, Examples of the method for producing a group III
nitride crystal substrate of the present invention are
described.
[0079] In following Examples, the flux solution growth using an
alkali metal as the flux is described. However, also in the case of
the group III nitride crystal produced by using the ammonothermal
method, by applying following Examples in the same manner as
described above, the impurities inside the crystal can be
removed.
[0080] In following Examples, Na was used as the alkali metal, and
the case where the group III nitride crystal is a GaN crystal is
described. However, the present invention can also be applied to
other group III nitride crystals such as AlN, AlGaN, GaInN and
AlGaInN.
Example 1
[0081] The process for producing the GaN substrate, as a group III
nitride crystal substrate, on the basis of the method illustrated
in FIGS. 1 and 2 is described as an example.
[0082] First, as shown in FIG. 2A, a GaN seed crystal was formed as
a group III nitride seed crystal 2 on a base substrate 1, by the
MOCVD method. For the base substrate 1, the sapphire (crystalline
Al.sub.2O.sub.3) (0001) C plane was used. Specifically, the base
substrate 1 was heated to about 1020.degree. C. to 1100.degree. C.,
trimethylgallium (TMG) and NH.sub.3 were fed onto the base
substrate 1, and thus the seed crystal 2 composed of gallium
nitride (GaN) was formed in a film thickness of 10 .mu.m. The GaN
seed crystal substrate, as a group III nitride seed crystal
substrate 3, was composed of the base substrate 1 and the seed
crystal 2.
[0083] Next, as shown in FIG. 2B, a GaN crystal, as a group III
nitride crystal 4, was grown on the seed crystal 2 (the crystal
growing step 11 in FIG. 1) by using the flux liquid phase growth
method. Hereinafter, an example of the flux liquid phase growth
method is described in detail.
[0084] First, Ga, Na, an alkali metal, as the flux, and Ge as an
n-type dopant, were weighed out in specified amounts (for example,
Ga:Na:Ge (molar ratio)=0.25:0.71:0.04), and were set in a crucible
together with the GaN seed crystal substrate, as the group III
nitride seed crystal substrate 3. Next, the crucible was placed in
a crystal growth vessel, and maintained at 800.degree. C. inside
the vessel to thereby convert the contents in the crucible into a
liquid phase; while the temperature and the pressure were being
maintained to be constant in a nitrogen gas atmosphere under a
pressure of 40 atm (40.times.1.013.times.10.sup.5 Pa), the liquid
phase growth was performed for 96 hours to yield the reaction
products including a GaN crystal.
[0085] Next, the crystal growth vessel was cooled back to the room
temperature, the contents of the crucible were converted into a
solid phase, and the crucible was taken out from the vessel. Then,
ethanol was placed in the crucible, the ethanol and the solid flux
covering the reaction products including the GaN crystal were
reacted with each other, and thus the flux was removed from around
the reaction products. Further, the reaction products were
subjected to an ultrasonic cleaning with purified water, and thus a
GaN crystal body, as the group III nitride crystal body 7 shown in
FIG. 2B, was obtained. The GaN crystal body had protruding
structures 9 on the periphery of the seed crystal and on the
periphery and the surface of the crystal body, due to the crystal
orientations and the differences in the shapes in the crystal
planes and in the peripheries of the crystals. In this way, the GaN
crystal body, as the group III nitride crystal body 7, was produced
by the flux liquid phase growth method.
[0086] Next, the GaN crystal body was fixed to a dummy substrate
with a wax by thermal adhesion and the protruding structures 9 were
removed by cylindrical grinding (FIG. 2C, the grinding step 12 in
FIG. 1).
[0087] Then, the substrate was subjected to beveling with a wafer
edge grinder, and the GaN crystal was sliced (cut) with a
multi-wire saw with reference to the underside of the base
substrate 1 so as to have a substrate thickness of 400 .mu.m to 600
.mu.m, to yield a GaN substrate, as a group III nitride seed
crystal substrate 5 (FIG. 2D, the slicing step 13 in FIG. 1).
[0088] Subsequently, the sliced GaN substrate was thermally adhered
with a wax to a dummy substrate, and then the GaN substrate, as the
group III nitride seed crystal substrate 5, was polished with a
polishing apparatus with reference to the underside of the base
substrate 1. By this polishing, the sliced GaN substrate was
subjected to parallel flattening to result in the substrate
thickness of 400 .mu.m.+-.40 .mu.m. Further, the parallel-flattened
substrate was mirror-polished with a single side polishing
apparatus, by varying the plate material, the load, the number of
rotation and the abrasive grain size to yield a GaN substrate, as
the group III nitride seed crystal substrate 6 in which the
Ga-surface roughness Ra of the GaN substrate was equal to or less
than 1 nm (FIG. 2E, the polishing step 14 in FIG. 1).
[0089] Subsequently, for the purpose of removing the wax used for
fixing the GaN substrate to the dummy substrate, organic cleaning
was performed. For the organic cleaning, solvent naphtha
(registered trademark), acetone, isopropyl alcohol and the like
were used. Further, for the purpose of removing the heavy metals
and the organic matter on the GaN substrate, the GaN substrate was
chemically etched with a concentrated sulfuric acid-containing
solution. Additionally, for the purpose of removing the oxide layer
of the GaN substrate, the GaN substrate was treated with buffer
hydrofluoric acid and dried with nitrogen (the first cleaning step
15 in FIG. 1).
[0090] Next, the GaN substrate was heat treated (the heat treating
step 16 in FIG. 1). First, under the condition that the GaN
substrate was placed in a quartz tube in a manner leaning against a
quartz boat, for the purpose of reducing the moisture and the
impurity gases such as oxygen in the furnace, a vacuum purge was
performed three times. Specifically, the following set of
operations was repeated three times: the pressure inside the
furnace was reduced to a pressure of -99 kPa or lower, and then the
pressure inside the furnace was made to be 1 atm
(1.times.1.013.times.10.sup.5 Pa) by feeding dry nitrogen into the
furnace. Next, while dry nitrogen was being made to flow at a flow
rate of 1 L/min (flow speed: 46 cm/min) and the pressure inside the
furnace was being made to be 1 atm (1.times.1.013.times.10.sup.5
Pa), the temperature was increased to 800.degree. C. at a
temperature increase rate of 200.degree. C./hr. Then, the
temperature was maintained at 800.degree. C. for 30 minutes, and
thus the temperature of the GaN substrate was made uniform.
Successively, the temperature was increased over 30 minutes to the
heat treatment temperature of 900.degree. C., and then the
temperature inside the furnace was maintained at 900.degree. C. for
90 minutes to perform the heat treatment. Then, the temperature was
decreased to room temperature at a rate of 200.degree. C./hr.
[0091] Finally, the GaN substrate after the heat treatment was
immersed in the buffer hydrofluoric acid solution for 10 minutes,
successively immersed in the sulfuric acid-hydrogen peroxide
mixture set at 120.degree. C. for 10 minutes, successively
subjected to running purified water cleaning for 10 minutes, and
then dried with nitrogen to produce a GaN substrate capable of
forming a semiconductor element (the second cleaning step 17 in
FIG. 1).
[0092] As described above, after the group III nitride crystal
substrate was formed by slicing the group III nitride crystal into
a wafer shape and before the epitaxial growth treatment, as an
element forming step, the heat treatment was performed at a
temperature equal to or higher than the lowest temperature capable
of vaporizing the flux and lower than the highest temperature at
which the surface of the group III nitride crystal substrate was
not decomposed, and thus the flux in the vicinity of the surface of
the group III nitride crystal substrate was able to be removed by
vaporization. Consequently, the failure due to the flux at the time
of the epitaxial growth was able to be suppressed.
[0093] Here, the case where the heat treatment temperature was set
at 900.degree. C. has been described; however, the heat treatment
temperature was also able to be set at, for example, 1000.degree.
C. or 1100.degree. C.
Comparative Example 1
[0094] As compared to Example 1, the heat treatment temperature was
altered to 800.degree. C., a temperature lower than the lowest
temperature capable of vaporizing the flux. The steps other than
the heat treating step were performed in the same manner as in
Example 1, and a GaN substrate was produced. The evaluation results
of the GaN substrate are described below.
Example 2
[0095] With reference to FIG. 3, the method for producing a group
III nitride crystal substrate of Example 2 is described by taking a
GaN substrate as an example.
[0096] Under the conditions that the heat treatment temperature was
set at 1200.degree. C., ammonia gas was made to flow at a flow rate
of 1 L/min (flow speed: 46 cm), and the pressure inside the furnace
was set at 1 atm (1.times.1.013.times.10.sup.5 Pa), the heat
treatment was performed for 2 hours, and thus, as described below,
asperities were formed on the surface of the GaN substrate after
the heat treatment. Accordingly, in present Example, the
improvement of the surface asperities was attempted by the
below-described "planarization."
[0097] As shown in FIG. 3, for the purpose of removing the
asperities on the substrate surface formed by the heat treatment,
the "planarizing step 18" was performed. It is to be noted that the
steps as far as the heat treating step 16 were the same as the
corresponding steps in the flow chart of Example 1 shown in FIG.
1.
[0098] As the "planarizing step 18," the surface of the substrate
was subjected to mechanical polishing or the like, the substrate
was cleaned to remove the wax used for the polishing, and the
substrate was subjected to a plasma dry etching using chlorine gas
for the purpose of removing the polishing-modified layer produced
by the mechanical polishing or the like. Finally, the "second
cleaning step 17" was performed in the same manner as in Example 1,
and thus the GaN substrate, as the group III nitride crystal
substrate, was obtained.
[0099] As described above, for the purpose of removing the
asperities, on the surface of the group III nitride crystal
substrate, produced by the heat treating step, in other words, for
the purpose of removing the group III nitride, low in the nitrogen
concentration, on the crystal substrate, the planarizing step 18 of
planarizing the surface of the group III nitride crystal substrate
was performed after the heat treating step 16. In this way, the
surface nature of the group III nitride crystal substrate was able
to be ensured, in addition to the fact that the failure due to the
flux, at the time of the epitaxial growth was able to be suppressed
by removing through vaporization the flux in the vicinity of the
surface of the group III nitride crystal substrate.
Comparative Example 2
[0100] As compared to Example 2, the heat treatment temperature was
altered to 1300.degree. C., a temperature higher than the limit
where decomposition occurred on the surface of the group III
nitride crystal substrate. The steps other than the heat treating
step were performed in the same manner as in Example 2, and a GaN
substrate was produced.
[0101] Hereinafter, description is made on the results of the
evaluations performed on the GaN substrates, being each an example
of the group III nitride crystal substrate, produced in
above-described Example 1, Comparative Example 1, Example 2 and
Comparative Example 2.
[0102] Here, the surface condition of a substrate after the heat
treatment was evaluated in terms of the surface roughness Ra after
the heat treatment and the macro-defect area ratio after the heat
treatment. The flux atomic concentration was evaluated by the
secondary ion mass spectrometry measurement.
[0103] The evaluation results of Examples 1 and 2 and Comparative
Examples 1 and 2 are described with reference to FIGS. 4 and 5 and
Table 1.
[0104] FIG. 4 is a graph showing the results of the secondary ion
mass spectrometry measurement for the case of the heat treatment
temperature of 900.degree. C. in Example 1, and FIG. 5 is a graph
showing the results of the secondary ion mass spectrometry
measurement for the case of the heat treatment temperature of
800.degree. C. in Comparative Example 1. Table 1 shows the
evaluation results of the group III nitride crystal substrates.
TABLE-US-00001 TABLE 1 Heat treatment (annealing) temperature
[.degree. C.] 800 900 1000 1100 1200 1300 Surface roughness G G G G
A P Ra of Ga-surface (after heat treatment) Macro-defect area G G G
G A P ratio (after heat treatment) Surface roughness A G G G *G --
Ra of Ga-surface (after epitaxial growth) Macro-defect area P G G G
*G -- ratio (after epitaxial growth) *Planarization was performed.
--: No evaluation was made.
[0105] The surface roughness was measured with a surface roughness
tester ZYGO (registered trademark), and was represented by the
surface roughness Ra (here, Ra is defined by the "arithmetical mean
roughness Ra") of the Ga-surface of the GaN substrate. First, the
surface roughness Ra after the heat treatment was measured, and
thus the surface condition depending on the heat treatment
conditions was evaluated. Further, as the evaluation standards for
the surface roughness Ra, from the viewpoint whether usable or not
for the epitaxial growth at the time of element formation, the case
where the surface condition was satisfactory was marked with "G
(Good)", the case where the surface condition was usable was marked
with "A (Average)" and the case where the surface condition was not
usable was marked with "P (Poor)". Here, the case of "G" is the
case where the surface roughness Ra after the heat treatment is 5
nm or less. A substrate marked with "G" can be used for the
production of a semiconductor device after the second cleaning step
as in Example 1. The case of "A" is the case where the surface
roughness Ra after the heat treatment exceeds 5 nm and is 10 nm or
less. In this case, as in Example 2, the planarization of the
substrate is preferably performed. By performing the planarizing
step and the second cleaning step, the substrate can be used for
the production of a semiconductor device. The case of "P" is the
case where the surface roughness Ra after the heat treatment
exceeds 10 nm. The case of "P" where Ra exceeds 10 nm is, as
compared to the usable case of "A", required to regulate the
operations and the time period of the planarizing step, hence the
step comes to be complicated to degrade the productivity, and the
case of "P" is not usable.
[0106] The macro-defect area ratio is described. The "macro-defect"
was defined as the defect discernible at an observation
magnification of 50 by using an optical microscope with an
objective lens having a magnification of 5. Specifically, the
observed image obtained with an observation magnification of 50 was
subjected to an image analysis to extract the macro-defects, and
the evaluation was performed in terms of the area ratio (%) of the
macro-defects to the total observation area. Examples of the
macro-defects include voids, foreign substances, crystal grain
boundaries, cracks and a secondary crystal phase (a crystal phase
other than the primary crystal phase). As compared to the clean
areas, these defects are different in the factors such as the
optical transmittance, the optical reflectance and the optical
phase difference, and hence these defects are discernible from the
normal clean areas by the observation with an optical microscope.
Here, the observation was performed in the light transmission mode,
the obtained observation image was binarized, the defect area was
obtained as the total areas of the individual defects, and the
ratio of the defect area to the whole observation area was derived
as the defect area ratio (%). The evaluation standards for the
macro-defect area are as follows: first the central area of the
substrate is defined by excluding the area extending from the
peripheral edge of the substrate to the location 2 mm inwardly away
from the edge; the case where the defect area ratio is 1% or less
in the central area is marked with "G (Good)"; the case where the
macro-defect area ratio exceeds 1% and is 50 or less in the central
area is marked with "A (Average)"; and the case where the
macro-defect area ratio exceeds 5% in the central area is marked
with "P (Poor)". A substrate marked with "G" can be used for the
production of a semiconductor device after the second cleaning step
as in Example 1. A substrate marked with "A" can be used for the
production of a semiconductor device by performing the planarizing
step and the second cleaning step as in Example 2. A substrate
marked with "P" is, as compared to the usable substrate marked with
"A", required to regulate the operations and the time period of the
planarizing step, hence the step comes to be complicated, and hence
even when an epitaxial film is grown to produce a device by using
this GaN substrate, the production yield comes to be low.
[0107] The surface nature after the epitaxial film growth was
evaluated similarly in terms of the surface roughness Ra and the
macro-defect area ratio of the epitaxial film, after the epitaxial
film was grown under the below-described conditions. The evaluation
standards for the surface nature are the same as described
above.
[0108] The conditions under which the epitaxial film was grown are
described. When the epitaxial film was grown, the MOCVD growth
method was used, the substrate temperature was set at 1050.degree.
C., TMG (trimethylgallium) gas and NH.sub.3 (ammonia) gas were used
as the main component gases, SiH.sub.4 (silane) gas was used as the
dopant gas, and thus a Si-doped n-type GaN layer having an n-type
carrier concentration of 5.times.10.sup.18 cm.sup.-3 to
1.times.10.sup.19 cm.sup.-3 was formed as a 2-.mu.m thick film on
the GaN substrate.
[0109] The secondary ion mass spectrometry measurement used for the
measurement of the flux atomic concentration is described. The
Na.sup.+ ion atomic concentration (atoms/cm.sup.3) was analyzed by
using CAMECA (registered trademark) -ims, under the measurement
conditions that the primary ion was O.sub.2.sup.+, the primary ion
energy was 8.0 keV, the primary ion current was 140 nA and the
analysis area diameter was 60 .mu.m, while the etching was being
performed from the GaN substrate surface to the depth of a few
micrometers. The Na.sup.+ atomic concentration, in the region from
the substrate surface to the depth of at least 3 .mu.m, is
preferably 1.times.10.sup.15 atoms/cm.sup.3 or less, and more
preferably equal to or less than 2.times.10.sup.14 atoms/cm.sup.3,
the detection limit of the secondary ion mass spectrometry
measurement under the above-described measurement conditions. The
Na.sup.+ ion atomic concentration exceeding 1.times.10.sup.15
atoms/cm.sup.3 in the region from the substrate surface to the
depth of 3 .mu.m offers a factor to cause short-circuiting in the
semiconductor device and thus affects the performances of the
device.
[0110] It is to be noted that the above-described examination of
various properties can be performed in the practical production
steps as an examination step in the same manner as described above.
This examination step is preferably performed after the heat
treating step 16.
[0111] As shown in Table 1, Example 1, Comparative Example 1,
Example 2 and Comparative Example 2 were classified according to
the heat treatment temperature, namely, the annealing temperature;
the cases the heat treatment temperatures of which were set
respectively at 800.degree. C., 900.degree. C., 1000.degree. C.,
1100.degree. C., 1200.degree. C. and 1300.degree. C. were
evaluated. The evaluation results are as follows.
[0112] (The Case of the Heat Treatment Temperature of 900.degree.
C.)
[0113] The surface roughness Ra of the Ga-surface of the GaN
substrate produced under this temperature condition was measured to
be Ra=0.6 nm so as to be marked with "G". Additionally, the
macro-defect area ratio after the heat treatment was found to be
0.03% so as to be marked with "G".
[0114] As is clear from FIG. 4 showing the results of the secondary
ion mass spectrometry measurement, the Na.sup.+ atomic
concentration had a lower value on the surface of the GaN substrate
than inside the substrate. This is because the Na remaining on the
surface of the substrate had been discharged to outside the
substrate by the heat treatment. Specifically, the Na.sup.+ atomic
concentration was 2.times.10.sup.14 atoms/cm.sup.3 on the surface
of the GaN substrate, was slightly increased from the surface to
the depth of 0.3 .mu.m, was saturated at the depth of 0.3 .mu.m or
more to be constant at 3.times.10.sup.14 atoms/cm.sup.3.
[0115] After the growth of the epitaxial film, the surface
condition of the GaN substrate was evaluated in terms of the
surface roughness Ra and the macro-defect area ratio. The surface
roughness Ra of the epitaxial film after the epitaxial growth was
somewhat increased, but was 1 nm to be marked with "G". On the
other hand, the macro-defect area ratio remained unchanged from the
surface condition before the epitaxial growth, to be marked with
"G". Additionally, the secondary ion mass spectrometry measurement
was performed to find the following: the Na.sup.+ atomic
concentration in the region from the vicinity of the interface
between the epitaxial film and the GaN crystal substrate to the
inside of the GaN substrate was, in the same manner as before the
epitaxial film growth, such that the Na.sup.+ atomic concentration
was small in the interface between the epitaxial film and the GaN
substrate, was slightly increased as far as the depth in the GaN
substrate was increased to be 0.3 .mu.m, and was saturated at a
depth of 0.3 .mu.m or more to be constant at 3.times.10.sup.14
atoms/cm.sup.3.
[0116] (The Case of the Heat Treatment Temperature of 800.degree.
C.)
[0117] The surface roughness Ra=0.5 nm and the macro-defect area
ratio 0.003% after the heat treatment at 800.degree. C. were both
satisfactory to be both marked with "G."
[0118] However, after the epitaxial growth, the surface roughness
Ra of the epitaxial film was 6.0 nm to be marked with "A".
Additionally, the macro-defects on the epitaxial film surface were
increased, and consequently the macro-defect area ratio was found
to be 5.6% so as to be marked with "P". The macro-defective
portions were recessed in shape (hereinafter, referred to as "pit
shape"), and recessed portions having a maximum depth of a few
hundreds nanometers were found to occur. The secondary ion mass
spectrometry of the macro-defective portions was performed in the
region from the vicinity of the interface between the epitaxial
film and the GaN crystal to the inside of the GaN substrate, and
found an abnormal deposition of sodium (Na), the flux component, in
the macro-defective portions inside the GaN substrate.
[0119] FIG. 5 shows the results of the secondary ion mass
spectrometry of the Na.sup.+ atomic concentration in the region
from the interface between the epitaxial film and the GaN substrate
to the inside of the GaN substrate, under this temperature
condition. Due to the epitaxial growth, the abnormal deposition of
Na.sup.+ atoms was found in the interface between the epitaxial
film and the GaN substrate, and the Na.sup.+ atomic concentration
in the vicinity of the interface was increased to be as high as
1.times.10.sup.19 atoms/cm.sup.3. On going from the interface deep
into the inside of the GaN crystal substrate, the Na.sup.+ atomic
concentration was abruptly decreased, exhibited a saturation
tendency at the depth of 500 nm from the interface, and came to be
an approximately constant value. In other words, the Na.sup.+
atomic concentration at the depth of 500 nm from the interface
exhibited a somewhat higher value of 1.times.10.sup.15
atoms/cm.sup.3 as compared to the Na.sup.+ atomic concentration
inside the GaN substrate in the case of the heat treatment
temperature of 900.degree. C. in Example 1. This is because, at the
time of the O.sub.2.sup.+ ion etching, Na.sup.+ ions reattached to
around the measurement region and then detached. In this case, the
Na.sup.+ ion offers a factor to cause short-circuiting failure in
the semiconductor device such as a laser diode or a light-emitting
diode formed with the epitaxial film, and thus adversely affects
the performances of the device. Additionally, the light output is
disadvantageously degraded as far as the long-term reliability is
concerned.
[0120] The abnormal deposition of Na occurring after the epitaxial
growth in the case of the heat treatment temperature of 800.degree.
C. is ascribable to the fact that the heat treatment temperature
was lower than the Na vaporization temperature of 883.degree. C.,
and hence the discharge of Na from the substrate through
gasification was not able to be performed. This abnormal deposition
is also ascribable to the fact that the epitaxial growth
temperature (1050.degree. C. to 1100.degree. C.) was higher than
the Na vaporization temperature of 883.degree. C., and consequently
the gasification of Na and the deposition of Na on the substrate
surface occurred at the time of epitaxial growth. When at the time
of the growth of GaN in the sodium flux liquid phase, the Na
contained in the defective portions in the vicinity of the surface
of the GaN substrate undergoes the change of state from liquid to
gas, the internal stress is generated with an increasing factor of
about 1000. The tensile strength of the normal portion of the GaN
substrate is 67 GPa, and is larger by a factor of about 2 than the
internal stress due to the vaporization of the flux although
depending on the flux amount. Accordingly, the generated internal
stress does not adversely affect the epitaxial film. However, the
defective portions of the GaN substrate are small in tensile
strength, and hence the internal stress due to the vaporization of
the flux exceeds the tensile strength of the defective portions, as
the case may be. In such a case, the surface of the GaN substrate
is roughened and exfoliated to adversely affect the epitaxial film
grown on the GaN substrate.
[0121] As described above, the evaluation results after the
production of the GaN substrate were satisfactory. However,
subsequently, due to the heat at the time of the epitaxial growth
for the formation of an element, the Na used as the flux was
deposited on the surface of the GaN substrate to offer causes for
failures. In other words, when the heat treatment temperature at
the time of the production of the substrate was set at 800.degree.
C., the flux was deposited at the time of the formation of an
element. Consequently, it has been revealed that when a
sodium-based flux is used at the time of the growth of a crystal,
the subsequent heat treatment temperature of 800.degree. C. cannot
be adopted.
[0122] (The Case of the Heat Treatment Temperature of 1000.degree.
C.)
[0123] When the heat treatment at 1000.degree. C. was performed in
Example 1, the surface roughness Ra of the Ga-surface was increased
to be 1.3 nm ("G"). The macro-defect area ratio after the heat
treatment was 0.01% ("G"); an observation with an electron
microscope at a magnification of 1000 identified the balloon-shaped
(droplet-shaped) defects formed by the deposition of Ga on the
surface and pit-shaped defects. When the epitaxial growth on the
GaN substrate was subsequently performed, the surface roughness Ra
was the same as the surface roughness before the growth of the
epitaxial film, namely, Ra=1.3 nm to be marked with "G", and no
increase of the macro-defects was identified and the macro-defect
area ratio remained to be 0.01% to be marked with "G".
[0124] In other words, when the heat treatment temperature was set
at 1000.degree. C. in Example 1, no abnormalities were found in the
epitaxial film on the GaN substrate after the epitaxial growth.
Consequently, it was verified that by setting the heat treatment
temperature at 1000.degree. C. in Example 1, the sodium (Na), the
flux component, was able to be effectively removed.
[0125] (The case of the heat treatment temperature of 1100.degree.
C.)
[0126] When the heat treatment at 1100.degree. C. was performed,
the surface roughness Ra of the Ga-surface of the GaN substrate was
1.5 nm ("G"). The macro-defect area ratio immediately after the
heat treatment was 0.15% to be marked with "G", and the pit-shaped
defects predominated the balloon-shaped defects. When the epitaxial
film growth on the GaN substrate was subsequently performed, no
increase of the surface roughness Ra and no increase of the
macro-defects were found (both of the surface roughness and the
macro-defect area ratio were marked with "G"). Consequently, it was
verified that by setting the heat treatment temperature at
1100.degree. C. in Example 1, the sodium (Na), the flux component,
was able to be effectively removed.
[0127] (The Case of the Heat Treatment Temperature of 1200.degree.
C.)
[0128] When the heat treatment was performed at 1200.degree. C.,
the surface roughness Ra of the Ga-surface of the GaN substrate was
8.3 nm ("A"), and the macro-defect area ratio of the GaN substrate
was 1.3% ("A"). Because the Ra exceeding 5 nm exerts adverse
effects at the time of the growth of the epitaxial film, according
to the prescriptions of Embodiment 2, the "planarization" was
performed after the heat treating step, in such a way that the
polishing amount was 1 .mu.m or less. To this planarizing step, a
mirror polishing using a diamond slurry having an average diameter
of 50 nm was applied. After the polishing, the dummy substrate was
detached, and for the purpose of removing the wax used for
adhesion, a cleaning step (the same treatment as the first cleaning
step 15) composed of solvent naphtha (registered trademark)
cleaning, acetone cleaning and isopropyl alcohol cleaning was
performed. Then, for the purpose of removing the
processing-modified layer due to the polishing of the GaN
substrate, a chlorine-based plasma dry etching was performed.
Further, the GaN substrate was subjected to the second cleaning
step in which the GaN substrate was immersed in the buffer
hydrofluoric acid solution for 10 minutes, subjected to running
purified water cleaning for 10 minutes, dried with nitrogen, then
immersed in the sulfuric acid-hydrogen peroxide mixture set at
120.degree. C. for 10 minutes and further subjected to running
purified water cleaning, and after the second cleaning step, the
GaN substrate was dried with nitrogen.
[0129] After the planarization and cleaning, the surface roughness
was measured again and the surface roughness Ra=0.5 nm was obtained
("G"). The macro-defect area was decreased by the planarizing step
to be 0.16% ("G"). In other words, when an epitaxial film growth
was performed on the GaN substrate produced by setting the heat
treatment temperature at 1200.degree. C. in Example 2, no
macro-defect increase was found. From this fact, it has been found
that even when the heat treatment temperature is as high as
1200.degree. C., a GaN substrate exerting no adverse effects on the
growth of the epitaxial film can be provided by removing the sodium
(Na), the flux component and by planarizing the GaN substrate.
[0130] (The Case of the Heat Treatment Temperature of 1300.degree.
C.)
[0131] By the heat treatment at 1300.degree. C., the surface
roughness Ra of the GaN was degraded to 22.3 nm ("P"), and the
macro-defect area ratio was also made to be 10.3% ("P") and the
balloon-shaped macro-defects were increased. This is ascribable to
the fact that by the heat treatment at 1300.degree. C., the
nitrogen in the GaN substrate was eliminated to decompose the GaN
crystal surface, and the Ga of the GaN crystal attached in a
droplet condition. In this case, it is necessary to remove the
balloon-shaped Ga generated at the time of the heat treatment,
before the planarizing step. However, even when the planarizing
step is performed under this condition, the step comes to be
extremely complicated, and hence the productivity is degraded.
[0132] From the above-described results, it has been verified that
after the group III nitride crystal substrate is produced by
slicing the group III nitride crystal grown by the flux liquid
phase growth method, by performing the heat treating step in an
nitrogen-containing gas atmosphere at a temperature which is equal
to or higher than the boiling point of the flux and at which the
group III nitride crystal is not decomposed, the flux exerting
adverse effects at the time of the growth of the epitaxial film can
be removed and thus a satisfactory group III nitride crystal
substrate can be provided.
[0133] In the description presented above, the case where GaN was
used as an example of the group III nitride was described; however,
the present invention can also be applied in the same manner to the
case where substrates are produced by using other group III
nitrides.
[0134] The case where the group III nitride crystal is produced by
the flux liquid phase growth method using an alkali metal has been
described; however, the present invention can also be applied in
the same manner to the case where the group III nitride crystal is
produced by the ammonothermal method using ammonia as the flux. In
the ammonothermal method, the present invention can also be applied
particularly to the case where an alkali metal or an alkali earth
metal is used as a mineralizer.
[0135] In the description presented above, described was the case
where the transmission mode of the microscope was used as the
method for evaluating the macro-defects. On the other hand, the
macro-defect amount may be evaluated by using a precise optical
scanner. Alternatively, the macro-defect amount may also be
evaluated from the light scattering amount of laser light. In the
evaluation of the macro-defects using a precise optical scanner or
laser light, it is preferable to examine the defects in the
thickness direction of the crystal as extensively as possible. For
that purpose, it is more preferable to examine the crystal by using
a precise scanner in the transmissive mode, or to examine the
crystal in an examination mode in which the laser beam reaches the
backside of the crystal in the case where an examination apparatus
uses laser scattering. When the reflection mode is used, it is
preferable to examine the macro-defects present, inside the
crystal, as far as the close proximity of the backside, by using
for example the light reflection on the crystal backside or the
like.
[0136] The sizes of the macro-defects are not particularly limited;
the sizes of the macro-defects are varied depending on the request
from the semiconductor device that uses the substrate. However,
preferable as the optical examination step is an examination step
capable of examining the whole surface of the crystal substrate in
a relatively short time and capable of examining macro-defects of
0.1 .mu.m or more.
[0137] When the macro-defects are quantified, it is preferable to
quantify, as the defect area ratio, the above-described ratio of
the macro-defective portion area to the whole examined area.
Example 3
[0138] By using the group III nitride crystal substrate obtained in
above-described Examples 1 and 2, a semiconductor laser diode was
produced as a semiconductor device. Description is made with
reference to FIG. 6. FIG. 6 is a sectional view illustrating the
structure of a semiconductor laser diode 90 of Example 3 of the
present invention.
[0139] First, an n-type GaN layer 92 having a film thickness of 2
.mu.m was formed on the surface of an n-type GaN substrate 91, to
which germanium (Ge) was added, produced by the methods of Examples
1 and 2. Silicon (Si) was added to the n-type GaN layer 92, by
using the MOCVD method (metal organic chemical vapor deposition
method) so as for the carrier concentration to be 5.times.10.sup.18
cm.sup.-3 or less (for example, 0.7.times.10.sup.18 cm.sup.-3).
[0140] Next, a clad layer 93 composed of n-type
Al.sub.0.07Ga.sub.0.93N and a light guide layer 94 composed of an
n-type GaN were formed on the n-type GaN layer 92. Next, a multiple
quantum well (MQW) composed of a well layer (thickness: about 3 nm)
composed of Ga.sub.0.8In.sub.0.2N and a barrier layer (thickness: 6
nm) composed of GaN was formed as an active layer 95. Then, a light
guide layer 96 composed of a p-type GaN and a clad layer 97
composed of p-type Al.sub.0.07Ga.sub.0.9N were formed. On the top
of the p-type clad layer 97, a ridge portion 97a to be a current
narrowing portion was formed. The semiconductor laser diode 90 is a
double heterojunction semiconductor laser, the energy gap of the
indium-containing well layer in the MQW active layer 95 is smaller
than the energy gap between the aluminum-containing n-type and
p-type clad layers 93 and 97. On the other hand, the light
refractive index is largest in the well layer of the active layer
95, and decreases in the order of the light guide layer 94, the
clad layers 93 and 97.
[0141] Then, on the whole surface of the clad layer 97, patterns
were formed by using the photolitho technique so as for the
lengthwise direction of the light resonator to lie in the
<1-100> direction, and thus an insulating film 99
constituting the electric current injection region having a width
of about 2 .mu.m was formed. Further, a portion of the insulating
film 99, on the top of the ridge portion 97a, was opened and a
contact layer 98 composed of a p-type GaN was formed.
[0142] Next, on the p-type contact layer 98, a p-side electrode 100
composed of Ni/Au and being in ohmic contact with the contact layer
98 was formed. Further, on the backside of the n-type GaN substrate
91, an n-side electrode 101 composed of Ti/Al and being in ohmic
contact with the n-type GaN substrate 91 was formed. Finally, by
cleaving in the (1-100) plane, the semiconductor laser diode 90
shown in FIG. 6 was produced. It is to be noted that the stripe
width of the laser was 1 .mu.m to 20 .mu.m, and the laser resonator
length was 500 .mu.m to 2000 .mu.m.
[0143] The device evaluation of the semiconductor laser diode 90
produced by the above-described method was performed. Specifically,
when to the obtained semiconductor laser diode 90, a predetermined
voltage was applied between the p-side electrode 100 and the n-side
electrode 101 in the forward direction, positive holes were
injected from the p-side electrode 100 into the MQW active layer
95, and at the same time electrons were injected from the n-side
electrode 101 into the MQW active layer. These holes and electrons
were recombined in the MQW active layer 95 to yield an optical
gain, and thus a laser oscillation occurred at an oscillation
wavelength of 404 nm. Further, for the purpose of observing the
defect density reduction effect, with a high output mode driven by
large current, the reliability of the semiconductor laser diode 90
was evaluated. The semiconductor laser diode 90 was continuously
operated at 25.degree. C. at a current density of 5 kA/cm.sup.2, as
the injection current density to give a laser power of 1 W, and
thus satisfactory results were obtained. Consequently, it has been
found that there can be provided a blue-purple semiconductor laser
being free from the effect of the flux component, being driven by
large current and having a high power mode.
[0144] As described above in respective Examples, when the flux
component having a boiling point lower than the epitaxial growth
temperature is mixed in the group III nitride crystal produced by
the flux growth method, the flux exerting adverse effects on the
growth of the epitaxial film can be removed, by heat treating the
group III nitride crystal, after the slicing step, in a
nitrogen-containing mixed gas atmosphere, at a temperature equal to
or higher than the lowest temperature (for example, the boiling
point of the flux) capable of discharging the flux from the crystal
and lower than the decomposition temperature of the group III
nitride crystal, and by subsequently cleaning the group III nitride
crystal. Consequently, by the present invention, stable device
properties can be realized. By performing the heat treatment based
on the present invention before the growth of the epitaxial film,
and by examining the surface of the group III nitride crystal
substrate, the impurities in the vicinity of the surface of the
group III nitride crystal substrate can be removed, and at the same
time, the presence and absence of the impurities exerting effects
at the time of the growth of the epitaxial film can be
inspected.
[0145] The group III nitride crystal substrate of the present
invention can be applied to various semiconductor devices such as
laser diodes, light-emitting diodes and field effect
transistors.
INDUSTRIAL APPLICABILITY
[0146] The method for producing a group III nitride crystal
substrate of the present invention can remove the flux exerting
adverse effects at the time of the growth of the epitaxial film
from the group III nitride crystal substrate produced by the flux
growth method. Consequently, the production method of the present
invention is useful for the method for producing a high-quality
group III nitride crystal substrate produced by the flux growth
method, the group III nitride crystal substrate, semiconductor
devices and others using the group III nitride crystal
substrate.
* * * * *